This invention relates to the removal of impurities such as iron oxides from molten salt baths, and more particularly, to their removal by magnetic means.
Certain metals, notably aluminum, magnesium and lead, are recovered from their oxides or halide salts by electrolytic reduction of the oxide or salt in a molten salt bath in which the metal oxide or metal salt is dissolved.
Unfortunately, however, certain other metal salts are usually present in the feed due to their occurrence together in nature and process economics involved in complete prior separation and purification.
Some impurities, such as silicon impurities in aluminum oxides or salts, are electrolytically reduced with the principal metal and may be later separated when high purity demands require such separation.
Other impurities, such as iron oxides present with salts of lead do not dissolve in the molten salt bath used for the electrolytic reduction of lead from lead chloride. While this may be desirable from the standpoint of purity of the reduced metal, it also results in an unacceptable buildup of iron oxide in the bath. If sufficient particulate is entrained in the liquid streams, it may cause attrition of graphite and refractory surfaces resulting in lowering of current efficiency, increased electrical current consumption per pound of metal reduced, shorting between electrodes and ultimately shutdown of the cell.
Furthermore, the presence of high concentrations of iron oxides increases the chance of chemical reaction between the iron oxide, carbon and chlorine in the cell to form iron chloride and carbon dioxide resulting in depletion of the carbon electrodes.
It would, therefore, be desirable to be able to periodically remove such buildups of iron oxide from a molten salt bath without necessitating shutdown and draining of the bath from the cell.